Digital data transmission system

ABSTRACT

A method is disclosed for transmitting super-rate data signals having a bit-rate higher than a predetermined bit-rate through a digital data transmission system normally providing channels having the predetermined bit-rate. The channels are subject to different propagation delay characteristics through the system. The method, comprises allocating a group of k channels in the data transmission system as a virtual channel for the transmission of the super-rate signals; dividing the super-rate signals into n sub-signals, where n≦k, having a bit-rate equal to or less than the predetermined bit-rate; generating delay calibration signals for transmission through the data transmission system; defining an overhead channel in at least one of the channels; transmitting the n sub-signals over the channels; transmitting the delay calibration signals in a rotational pattern over the channels in slots normally containing data signals, the delay calibration signals temporarily displacing the data signals normally occupying the slots; transmitting the displaced data signals over the overhead channel in slots normally occupied by the delay calibration signals displacing them; and reassembling the n sub-signals to reconstitute the original super-rate data signals with the aid of the delay calibration signals after transmission through the channels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for transmitting data over adigital transmission system having discrete data channels with apredetermined base bit-rate lower than the bit-rate of the data to betransmitted. More particularly, the invention provides a method andapparatus for allocating a number of base rate channels to a virtualchannel capable of carrying signals having a bit-rate higher than thebase rate.

2. Description of the Prior Art

One data transmission system to which the invention is especiallyapplicable is the Integrated Services Digital Network (ISDN) for whichthe standards have been defined by the International Telegraph andTelephone Consultative Committee (CCITT). ISDN makes use of a 2.048megabyte per second primary rate TDM channel onto which aretime-division-multiplexed 32 sub-channels (DS0). Each subchannel has abase bit-rate of 64 Kb per second. The primary rate TDM channel carries8000 frames per second, each divided into 32 time slots carrying onebyte (8 bits) of data from each sub-channel. The first time slot in eachframe is used to identify the start of the frame, and another time slot,which constitutes the D channel, carries routing instructions. Theremaining 30 slots are available for carrying data, normally as 30discrete channels.

The primary rate TDM channel might, for example, be used to connect aprivate branch exchange (PBX) to the public telephone network. A singleprimary rate channel will therefore give the subscriber access to thirty64 Kbps base rate channels. Situations often arise, such as in thetransmission of image and video signals, or a high volume of computerdata, where it is desirable for the subscriber to transmit the data at arate higher than the base rate. For example, it would be desirable tohave the capability of sending a 128 Kbps bit stream over two parallel64 Kbps base rate channels. Unfortunately, because of the switchingrequirements and propagation characteristics of the public network, achannel which occupies a particular time slot in any given frame of thetransmitted signal does not necessarily occupy the same time slot at thefar end. The base rate channels are subject to different delays throughthe network. As a result, if the channels are merely reassembledsequentially at the far end, the transmitted data is scrambled andunusable.

International Patent Application No. WO 85/04300 describes a systemwherein prior to data transmission synchronization signals aretransmitted along each of the base channels to determine the delaysapplicable to each channel. A reframer unit then takes into accountthese delays to reassemble the transmitted data in the correct order.The problem with this system is that once the virtual channel has beenestablished it cannot be changed without being completely reset.Furthermore if the delays for the various channels change duringtransmission, the data becomes unusable. The system cannot therefore beregarded as reliable.

U.S. Pat. No. 4,805,167 describes a system for providing a variable datarate aggregate channel. In this system marker signals are sent on thebase rate channels to specify the order of transmission of thesub-signals to as to ensure correct reassembly at the far end. Oneproblem with this system is that it can only be used for packettransmission since it requires there to be idle time slots in the datachannels to carry the marker signals. It cannot therefore be used withcontinuous signals, such as video signals because there are no vacantslots in which to insert the marker signals. Also, since the markersignals can only be inserted when idle time slots are available, it doesnot permit the delay characteristics of the network to be continuallymonitored.

An object of the invention is to provide an improved transmission systemwhich does not depend on vacant slots being available in the data streamand which permits continuous monitoring of the channel delays.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof transmitting signals having a bit-rate higher than a predeterminedbit-rate rate in a digital data transmission system normally providingdiscrete data channels having said predetermined base bit-rate, saidchannels being subject to different propagation delay characteristicsthrough the system, comprising allocating a group of k saidpredetermined bit-rate data channels as a virtual channel for thetransmission of said high bit-rate signals, characterized in that anoverhead channel is defined in at least one of said group of channels,said high bit-rate signals are divided into n sub-signals, where n≦k,having a bit-rate equal to or less than said predetermined bit-rate,said n sub-signals are transmitted over said data channels, overheadsignals are transmitted at intervals over said channels in slotsnormally allocated for data, said data slots being transmitted over saidoverhead channel while said overhead signals are transmitted in theirplace, and said overhead signals are used at the far end to reassemblesaid n subsignals into said original high bit-rate data signal.

In one embodiment, k=n+1 and the extra channel is dedicated exclusivelyas the overhead channel.

In systems where the aggregate rate is low, the expense of a 64 Kbpsoverhead channel is excessive. In this case, a group of C₀ . . . C_(n-1)predetermined bit-rate data channels are allocated as a virtual channel.One of the channels C_(m) has a free bit position not required fortransmitting data and provides the overhead channel at a subrate of thepredetermined bit-rate. The overhead signals are transmitted byinserting respective individual bits thereof into the respective datachannels, each bit of the overhead signals being inserted into the freebit position of channel C_(m) or into data bit positions of the othern-1 channels. The data bit that would normally be sent in a bit positionoccupied by an overhead signal bit is sent in the free bit position ofthe channel C_(m) while its bit position is occupied by an overhead bit.

In a preferred embodiment the overhead signals, which may comprise delaycalibration signals, are transmitted successively over the respectivechannels in a rotational pattern. For example, in the case of a virtualchannel consisting of four data channels and one overhead channel, ittakes successive frames to complete one rotation. In a first frame, thedelay calibration signals are transmitted in the time slot correspondingto the overhead channel. In a second frame the delay calibration byte istransmitted in the time slot corresponding to the first data channel andthe data that would normally be sent in that time slot is instead sentin the time slot in the overhead channel. Likewise in the third frame,data from the second channel is swapped into the overhead channel timeslot and the delay calibration signal sent in its place, and so on untilafter the delay calibration signals have been sent in the fourth datachannel the next delay calibration signals are sent in the overheadchannel, whereafter the cycle is repeated.

Each data channel (DSO) may be delayed differently by the network. Thedelay calibration signals, hereafter referred to as delay calibrationbyte (DCB), form a framing pattern in each channel which can beextracted at the receiver. The contents of the DCBs permit the relativedelay to be determined. In the scheme just described, every end byte onthe far end channels constitutes the framing pattern. The rotatingcalibration scheme may be performed continuously throughout thetransmission, or used initially to establish superrate transmission andthen discontinued.

To determine the relative delay between channels the overhead byte sendsout a rotation count (LSB). A rotation starts with a DCB sent out on theoverhead channel and ends when the last data channel in the virtualchannel has sent its DCB. At the receiver this creates the appearance ofevery (N+1)th byte on the channel forming a framing pattern.

To accommodate 48 Kbps data channels, only the six most significant databits of each byte are used. The remaining bits are set to "1" to ensurecorrect 1's density on Switch 56 networks. Six bits provide 64 uniquebytes for the DCBs, but since the count wraps around only half therotation count can be resolved. One byte rotation count allowsdifferential delays of 32 frames to be detected. Since the DCB wouldoccur every 2N overhead slots the maximum delay resolution would be2N×32 frames, which in practice is insufficient.

To overcome this problem, a second slot is used to send the mostsignificant bits (MSB) of the rotation count, which results in a 12 bitdelay calibration count. This permits 2048×2N frames to be resolved. Inthe worst case, where the channel size is three (N=3) this results in12,228 frames, which is equal to 1.528 seconds. This is sufficient formixed terrestrial satellite networks.

The overhead slots are used for the LSB count, the MSB count and generalpurpose overhead information. The LSB appear every second overhead slot.The slots not used for LSB alternate sending the MSB and an overheadbyte. The MSB follows even LSB counts and the OHB (overhead byte) slotfollows odd LSB slots.

Another aspect of the invention provides an apparatus for transmittingsignals having a bit-rate higher than a predetermined bit-rate rate in adigital data transmission system normally providing discrete datachannels having said predetermined base bit-rate, said channels beingsubject to different propagation delay characteristics through thesystem, comprising means for allocating a group of k said predeterminedbit-rate data channels as a virtual channel for the transmission of saidhigh bit-rate signals, an overhead channel being defined in one of saidchannels, means for dividing said high bit-rate signals into nsub-signals, where n≦k, having a bit-rate equal to or less than saidpredetermined bit-rate, means for transmitting said n sub-signals oversaid n data channels, means for transmitting overhead signals atintervals over said channels, the overhead being transmitted in saiddata channels in slots normally allocated for data, said data slotsbeing transmitted over said overhead channel while said calibrationsignals are transmitted in their place, and means at the far end forusing said delay calibration signals to reassemble said n subsignalsinto said original high bit-rate data signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIGS. 1a and 1b are a schematic diagram of a virtual channel operatingin accordance with the invention;

FIG. 1c shows the scheme for sending DCBs in the successive time slotsof a single DSO data channel;

FIG. 2 is a block diagram of a data transmission system incorporation avirtual channel in accordance with the invention;

FIG. 3 is a flow chart illustrating the operation of a frame state unit;

FIG. 4 is a diagram illustrating the consequences of a base channelslipping within a virtual channel;

FIG. 5 is a simplified block diagram of a four-node transmissionnetwork;

FIG. 6 is a block diagram of two-node network; and

FIG. 7 is a diagram showing the transmission scheme in accordance with asecond embodiment of the invention, where the overhead channel isdefined by a vacant bit position within a data channel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described with reference to an ISDN network havinga primary rate TDM carrier of 2.5048 Mb per second carrying thirty datachannels and two system channels. As discussed above the primary ratechannel carries 8000 frames per second, each base channel (DSO) having abase rate of 64 Kb per second.

FIGS. 1a and 1b show respectively a transmit section and a receiversection in communication through a public telephone network. It shouldbe noted that while for convenience the two sections are designated asthe transmitter and receiver respectively, the system operates in thefull duplex mode. The system is symmetrical and either section can serveas transmitter or receiver.

A 256 super-rate Kbps bit stream 1 is divided into four parallel bitstreams 2, 3, 4, 5 each running at 64 Kbps (256÷4=64). The four bitstreams 2, 3, 4, 5 are applied to a Virtual Channel Resource (VCR) unit6, in the transmit section, which outputs four base rate bit streams 7,8, 9, 10 serving as data channels and an additional base rate bit stream11, which normally serves a data channel, but which in the virtualchannel scheme serves as an overhead channel.

The five base rate bit streams are transmitted over five base rate (DSO)channels on the public telephone network to a Virtual Channel Resource(VCR) unit 12 in the receiver section. VCR unit 12 includes delaybuffers 13 respectively receiving each of the incoming channels and areframer unit 14 for reassembling the incoming bytes in their propersequence so as to reconstitute the four original bit streams as 2', 3',4', 5'. FIG. 3 is a flow chart showing the operation of the receiverframing unit 14.

The reconstituted bit streams are then recombined into the original 256Kbps super-rate bit stream.

The VCR transmit unit 6 inserts delay calibration bytes (DCB) 15 amongthe data bytes 16 according to the scheme shown. In frame 1, the firstdelay calibration byte (DCB) is inserted into the time slotcorresponding to the overhead channel. In the next frame, the DCB isinserted in the time slot corresponding to the first data channel 7 andthe contents of this time slot are transmitted in the time slotcorresponding to the overhead channel. In frame 3 the contents of thetime slot corresponding to the second data channel 8 are transmitted inthe overhead channel, and the DCB is transmitted in this time slot. Thescheme is repeated in the third and fourth data channels 9 and 10,whereafter in the sixth frame the DCB is transmitted in the overheadchannel time slot, and the pattern is repeated on a rotational basis.

FIG. 1b shows how the bytes constituting the DCBs (LSBs, MBs, and OHBs)are sent successively in the DCB slots of a single DSO channel. Thefirst LSB, representing the rotation count, is sent in the first DCBslot. There then follows three regular data slots followed by the MSB inthe next DCB slot. The next DCB slots contain successively the LSB=1byte, the OHB, the LSB=2 byte and the next MSB.

FIG. 2 shows the hardware implementation of the virtual channel. DataTermination Unit (DTU) 20 of the transmit section T receives theincoming super-rate bit stream at 256 Kbps and outputs four parallel 64Kbps bit streams on ports 0, 1, 2, and 3. These bit streams areconnected by switch 21 to the Virtual Channel Resource (VCR) transmitunit 6 in such a way that the lower circuit number on the datatransmission unit 20 is connected to the lower circuit number at theinput of the VCR transmit unit 6. The VCR transmit unit 6 outputs fivechannels (four data channels+one overhead channel) to switch 22, whichis in turn connected to the line termination transmit unit 23. This isconnected via a time division multiplex line 24 forming part of thepublic digital transmission network to the line termination receiverunit 25 of the receiver section R. The receiver unit 25 outputs fivechannels (including the overhead channel) to the switch unit 26, whichis connected to the VCR receiver unit 12. This extracts the overheadinformation and outputs four data channels to the switch unit 27. Theswitch 27 connects the lower incoming circuit number from the virtualchannel resource receiver unit 12 to the outgoing lowest circuit numberon the DTU 28. This in turn outputs the original super-rate 250 Kbpsbits stream.

Although the invention has been described in connection with a 64 Kbsbase rate network, in order to accommodate 48 Kbps data channels, whichare used in some countries such as the United States, only the six mostsignificant data bits of each byte can be used according to the schemedescribed above.

A rotation starts with a DCB sent out on the overhead channel 11 andends when the last data channel 10 in the virtual channel has sent itsDCB, i.e. at frame 5 in FIG. 1. At the receiver R this creates theappearance of every Nth channel forming a framing pattern.

The Overhead Bytes (OHB) are described below. As discussed above, theseare sent in the DCBs alternating with the LSB rotation counts. The slotfollowing rotation count LSB=0 is used for rotation count MSB and maynot be used for overhead information.

The Overhead Bytes are six bits to allow for 48 Kbps transfer. Overheadinformation is sent via a one or two byte pattern. The OHBs aredescribed below. The unused bits are set to 1 to comply with Switch 56ones density requirements. The overhead bytes consist of:.

Status Words

These are used to advise the sender of the status of an incoming virtualchannel.

    ______________________________________                                        00abcd11   Channel Status Word                                                           a = virtual channel synch (1 = in synch)                                      b = virtual channel state bit 1                                               c = virtual channel state bit 1                                               d = data synch (1 = in synch)                                               State Virtual Channel State                                          ______________________________________                                                 00    In Service                                                              01    Calibrate and leave                                                     10    Out of Frame                                                   ______________________________________                                    

Channel ID Bytes

These are used to indicate the channel/virtual channel numbers of thetransmitted virtual channel (VC) to the receiver.

    ______________________________________                                        01nnnn11  nnnnn =                                                                            0 following byte is channel #LSB                                              1 following byte is channel #MSB                                              2 following byte is VC #LSB                                                   3 following byte is VC #MSB                                                   4 channel mode = start up mode                                                5 channel mode = continuous mode                                              6 channel mode = transparent mode                              ______________________________________                                    

The following overhead byte codes may only be sent in the overheadchannel (channel number 0):

    ______________________________________                                        10nnnn11  nnnn = 0                                                                            overhead channel message start                                                1 overhead channel message end                                11abcd11  abcd = >                                                                            ABCD signalling bits                                          ______________________________________                                    

In a given 100 ms interval each channel must send the OH bytes listedbelow one or more times.

channel status word

channel number

virtual channel number

channel mode

Overhead channels in message mode are exempt from this requirement.

In order to correct delay at the receiver the sequence in which therotation count is sent and the assignment of channel number to circuitnumbers must be known by the receiver. The channel rotation count issent first on the overhead channel and then on the data channels inorder of increasing channel number. The channel numbers are assigned tothe sender data circuits in the following fashion, which is given by wayof example with reference to a Newbridge 3645 Mainstreet system. Thechannels must be numbered in some fixed manner which is the same at eachend:

in an ST-BUS the earlier timeslots get the lowest numbers

in MX streams simultaneous ST-BUS timeslots are ordered by PE slotnumber

in MX links the timeslots are ordered D1<D2<CB1<CB2

This protocol requires that data circuits from the data source (DTU) 20to VCR transmit unit 6 be mapped in the same fashion as the circuitsfrom the VCR receive unit 12 to data receiver unit 28. Switches 21, 22,26, 27 bring this about, and this arrangement ensures that datainterface ports are properly connected.

The circuits between the VCRs need not be consistently mapped. The VCRreceive unit 12 will determine the channel number assignments and sendthe data out in increasing timeslot order on the data circuits to thedata receiver. This allows independence of the network.

In the continuous calibration mode, discussed below, the overheadchannel (channel number 0) may be used to send messages from end to end.The messages are sent in the overhead DCB slots following an overheadchannel message start byte pattern. The overhead channel on byte patternis repeated 10 times before the message channel is active. Once thechannel is active a message of not more than 128 bytes may be sent. Themessage is terminated by sending overhead channel message end byte(repeated 10 times). While the overhead channel is active no othermessages may be sent on channel 0.

The overhead message packets are not defined. Any suitable packet basedprotocol can be used to send messages. They are used for messages for:

Performance summary (slips, LSB count errors etc.)

Dynamic channel sizing

In order to accommodate a variety of network types several modes exist.All of these modes assume that the virtual channel is full duplex. Thechannel mode is sent in overhead bytes in all channels. Examination ofany one channel is sufficient for determining the mode. The mode shouldbe debounced for 4 super-rotations.

1. Continuous Calibration Mode

Data and DCB patterns are inserted at start-up and continued for thelife of the channel. For transport of N data channels N+1 circuitsbetween VCRs must be allocated. The overhead channel should be used tocontinuously send the following information:

Channel Number (LSB & MSB)

Virtual Channel Number (LSB & MSB)

Data Channel Status

Continuous Mode Channel

2. Start-up Calibration Mode Only

This mode exists to allow overhead-free transmission of super-rate datafollowing initial set-up. In this case for N data circuits only Ninter-VCR circuits are required. These channels are numbered from 1 toN. This mode starts with the transmission of the standard rotating DCBpattern. The overhead DCBs send (continuously):

Start-Up Mode Channel

Channel Number LSB

Channel Number MSB

Far End Channel Data Status

Far End Virtual Channel Status

To simplify framing at the far-end all ones are sent in place of data.Once the receiving VCR has framed and extracted the necessaryinformation from the overhead bytes, it sends a data synch=1 in itsChannel Status Word, and virtual channel synch=1 and state=In Service inthe Virtual Channel Status Word.

The sending VCR upon receipt of virtual channel synch sends a transferto transparent signal in the data status word of channel 1. This signalconsists of 8 repetitions of the Data Status Word in successive OH byteswith mode set to Transparent. Once the signal is complete the senderthen shifts to transparent mode.

3. Transparent Mode

This mode allows for direct super-rate virtual channel connectionwithout continuous delay calibration. This mode can be startedimmediately, or entered after a delay calibrating start-up sequence.While in the transparent mode the receiver continues to monitor for aframing pattern. If framing is detected and maintained for 500 ms on allof the channels forming a virtual channel then the receiver change modesto correspond to the mode type in the data status word of the incomingdata.

Ideally the maximum reframe time for a single DSO within a virtualchannel should be 500 ms, so the maximum time to re-calibrate an entirevirtual channel would be 2.5 secs. The maximum time to correct a frameslip on a DSO would be 100 ms. For large values of N, these targets maynot be attainable.

Channel Operation

In normal operation each channel sends data status and virtual channeloverhead bytes. During start-up the channel watches for far end VirtualChannel synch. If synch is not received within 10 secs, a recoverysignal is broadcast. If VC synch is received within 10 secs the channeldeclares In-service and continues monitoring or moves to the transparentmode.

In the event of a data channel out-of-frame the receiver sends anout-of-synch signal in the channel status byte and all other channelswithin the virtual channel send Virtual Channel Out-of-Synch (VCOS) intheir virtual channel status words. If VCOS is detected in the incomingstreams and persists for 5 secs, the sender declaresFar-End-Out-Of-Frame and enters the recovery mode. If the receivercannot frame on the incoming stream within 5 secs, it declaresNear-End-Out-Of-Frame and continues to maintain far end alignment.

Frame slips occur as a result of asynchronism between network elements.A frame slip will result in either the duplication of deletion of aframe of information within the transmission facility. If a singlechannel within a virtual channel slips then it will become permanentlymisaligned with the other members of the virtual channel as shown inFIG. 4. In the virtual channel information the data all starts withinone synchronization domain. This leads to two slip scenarios:

phase error slips due to an asynchronous network span

continuous slips due to different network synch sources

The phase error slip case results in a maximum delay adjustment of +/-1frame. In FIG. 5, which relates to the four node case and FIG. 6, whichrelates to the two node case, node 2 runs slightly slower than thenetwork. Data from 1→2 arrives too quickly and the receiver mustperiodically throw away a frame of data. At the 2→3 interface data isarriving too slowly and a frame must be duplicated occasionally tocorrect. The net effect on the channel is that it will periodically slipin one direction and then eventually slip back. Each time the slipoccurs the delay of the affected channel must be changed.

In the continuous slip case, slips occur in one direction only. Theslips on links A & B will be out of phase. If an attempt is made tocompensate by changing delay on the channel which slips, then as slipsaccumulate so will the delay. Eventually the buffer space for delayingwill be exhausted and it will be necessary to disrupt service tore-calibrate. One solution, provided a knowledge of network topology isavailable, is to add a delay to channel 1 when link 1 slips and removethe delay from channel 1 when link 2 slips, in which case the need tore-calibrate can be avoided.

A frame slip will misalign data on the virtual channel. The slips shouldbe detected and corrected quickly. At the receiver the framing patternwill slip, and the framer must cope with slips without declaring out-of-frame.

The receiver framing operation will now be described in more detail. Atthe base-rate channel (DSO) level the rotating DCB pattern appears as aDCB every N frames. This allows each DSO to be considered as anindependent channel which must frame on the incoming DCB pattern. Delayequalization is then performed by reading the current DCB count and thebyte offset from it from each DSO in a known time-sequence. This allowsthe relative delay between the DSO to be calculated and the variabledelay buffers changed. It is permissible to allow the DSOs to reframe asa consequence of delay adjustment provided that the performanceobjectives are met.

The start-up time and the recovery time from protection switches dependson the time it takes the receiver to frame on each DSO in the virtualchannel. The frequency of delay calibration bytes is a function of thevirtual channel size. As the frequency of DCBs decreases reframe timeincreases.

The VCR state framer, for which the flow chart is shown in FIG. 3,handles slips without re-framing to allow very fast slip response time.In the start state the framer selects a byte from the data stream andmoves to the Load First DCB state. N data bytes later the second DCB isloaded on the transition Load Second DCB state. If the two DCBs form avalid pattern (the second is one more than the first) then the ReverseGuard state is entered (Note: if the first DCB was 0 then the followingDCB is a MSB and the one after that is an overhead byte, in this case avalid pattern cannot be declared until the next LSB rotation count isexamined). From Reverse Guard the pattern is checked until GUARD-COUNTcorrect DCBs have been detected at which point an in-frame is declaredand the Forward Guard state entered. A 2/4 DCB error causes loss offrame and transition to Load Second DCB. In addition to looking in theexpected timeslots, the forward guard state examines the timeslot oneither side. If a valid pattern is detected between the previoustimeslot DCB and one on either side of the present DCB then a Watch forSlip state is entered. If GUARD-COUNT valid DCB bytes are observed inthe +/- 1 timeslot then slip is declared, the timeslot for examiningframing is moved and SLIP is declared. If 2/4 DCB error in timeslot +/-1 are encountered in one of the Wait For Slip states then the LoadSecond DCB state is entered.

The parameter GUARD-COUNT is a feature of the framer. It should beadjustable on a per Virtual Channel Resource level. The ability toadjust GUARD-COUNT on a per virtual channel basis is desirable, but notrequired. The frequency of DCB bytes (rotation time) is N×125 μsec. Thistime is calculated for various channel sizes in the table below:

    ______________________________________                                        N         Rotation Time                                                                             Super-Rotation Time                                     ______________________________________                                         8        1 ms         256 ms                                                 16        2 ms         512 ms                                                 32        4 ms        1024 ms                                                 64        8 ms        2048 ms                                                 128       16 ms       4096 ms                                                 256       32 ms       8192 ms                                                 ______________________________________                                    

The overhead frequency is 2×Rotation Time. The framer slip detectiondepends on the size of channel and the framer parameter GUARD-COUNT.Some times for various values of parameters are indicated below:

    ______________________________________                                        N             G C    Slip Response                                            ______________________________________                                         8            4      10 ms                                                     8            8      18 ms                                                     32           4      20 ms                                                     32           8      36 ms                                                    256           2      192 ms                                                   256           4      320 ms                                                   256           8      576 ms                                                   ______________________________________                                    

These numbers assume GUARD-COUNT+1 rotation count LSBs are required fordeclaration of slip. Time to implement the delay adjustment is notincluded.

In systems where the aggregate rate is low, the expense of a 64 Kbpsoverhead channel is excessive. In accordance with the embodiment shownin FIG. 7, the overhead channel is formed by inserting overhead bitsinto data (DSO) channels and temporarily inserting the data bitsnormally occupying the bit positions into a spare bit position in one ofthe data channels.

Referring now to FIG. 7, this shows an 8 Kb per second virtual channelprotocol. Each channel has eight bits, numbered 0 to 7, with 7 being thefirst bit time slot (this is different from conventional telecompactors,where the bits are numbered 1 to 8, with 1 as the first time slot).

FIG. 7 shows three channels C₀, C₁, C₂, which are to be used forsuper-frame transmission. Each channel is a 64 Kbps DSO base ratechannel, and the channels are time division multiplexed in a knownmanner. In order to transmit data through the virtual channel at a bitrate higher than the bit rate of DSO channels (super-frametransmission), overhead information has to be transmitted through thenetwork in an overhead channel. This is an 8 Kbps virtual channel formedby substituting overhead bits in predetermined bit positions in rotationin the data channels.

As shown in FIG. 7, in frame 0 overhead bit 1 is inserted in bitposition 2 of channel C₀. Channel C₁ and C₂ are unaffected. In frame 2,the data bits 2 that would normally be transmitted in bit position 2 ofchannel C₁ is transmitted in bit position 2 of channel C₀ while theoverhead bit 1 is temporarily substituted. Channel C₂ is unaffected. Inthe next frame data bit 3, that would normally be located in bitposition 2 of channel C₂ is inserted in bit position 2 of channel C₀while overhead bit 1 is inserted in its corresponding position inchannel C₂.

This rotational sequence then continues, the substitution of theoverhead bits thus allowing the formation of an overhead channel withinthe data channels without the need to use a separate data channel foroverhead purposes. The effect on any given channel therefore is theappearance of overhead bits every third byte. In the more general case,where N channels are aggregated, the overhead bit will appear everyN^(th) byte. Six types of overhead bits exist as follows:

framing bits

delay calibration timestamp bits

status bits

mode bits

super-frame bits

The format of these bits will now be described in more detail asfollows:

Framing Structure

The VCP8 super frame is shown below:

    __________________________________________________________________________    MD <d> F1 <d> DC5 <d> F0 <d> CID5 <d> F0 <d> ST5 <d> F1 <d>                   MD <d> F1 <d> DC4 <d> F0 <d> CID4 <d> F0 <d> ST4 <d> F1 <d>                   MD <d> F1 <d> DC3 <d> F0 <d> CID3 <d> F0 <d> ST3 <d> F1 <d>                   MD <d> F1 <d> DC2 <d> F0 <d> CID2 <d> F0 <d> ST2 <d> F1 <d>                   MD <d> F1 <d> DC1 <d> F0 <d> CID1 <d> F0 <d> ST1 <d> F1 <d>                   MD <d> F1 <d> DC0 <d> F0 <d> CID0 <d> F0 <d> ST0 <d> F1                       __________________________________________________________________________    <d>                                                                       

where

<d> represents N-1 frames of data bits.

Fx--framing bit (value=x)

SFx--super frame bit (value=x)

DCn--bit n of delay calibration timestamp

CIDn--bit n of the channel ID

STn--bit of the status word

MD--is the channel mode (0=transparent, 1=monitor)

The super frame consists of six frames, each consisting of eightoverhead bits. four of these overhead bits form the basic 1001 framingpattern. The remaining four bits per frame are used to carry statussignals. Since four bits per frame is not enough to carry the requiredoverhead information a group of six frames are used to construct asuper-frame. The super-frame alignment bits are carried in the firstcolumn of the super-frame.

Delay Calibration Timestamp

Each super-frame carries a (Delay Calibration Timestamp) DCT. The sourceof the virtual channel protocol sends a continually incrementing 6 bitvalue in this field. The DCT value is incremented after each super-frameof overhead has been sent.

The delay resolution provided by this mechanism is a function of virtualchannel size. This will be discussed further in the performanceestimates section.

Channel ID

The channel ID field provides a 6 bit channel identifier, which resultsin a maximum virtual channel size of 64.

Status Word

The six bit status word is defined as:

    ______________________________________                                              bit 5  Signal Bit D                                                           bit 4  Signal Bit C                                                           bit 3  Signal Bit B                                                           bit 2  Signal Bit A                                                           bit 1  Virtual Channel Synch (VSYNCH) 1 = in                            synch                                                                               bit 0  Channel Data Synch (DSYNCH) 1 = in synch                         ______________________________________                                    

DSYNCH--each channel in the Virtual channel will use the DSYNCH bit tosend its framer status to the far end. If it is In Frame it will setDSYNCH=1, otherwise DSYNCH=0.

VSYNCH--each channel in the virtual channel will send VSYNCH=1 if thevirtual channel is In Service. An In Service Virtual Channel is definedas one for which:

all channels have data synch

all delay calibration has been performed

Signalling bits are defined only for the channel with the lowest channelnumber in a virtual channel group (typically channel 0). The signallingbits on other channels in the group are undefined.

Far End Status

Far end data synch is known from the DSYNCH bit directly

Far Enc Virtual Channel Synch is assumed if ALL channels send VSYNCH.

If channels are out of frame at the near end then their status bitscannot be extracted and should be ignored. The remaining VSYNCH bits areused to determine the status of the virtual channel. If a channel showsVSYNCH for 10 ms then it can be declared In Service.

Delay Determination

The receiver can determine the relative delays between channels bykeeping the following information:

DCT count from past super-frame

frame offset from the beginning of the present super-frame

channel ID

The DCT difference can only be resolved to half of the DCT size (due towrap-around). Prior to performing any calculations the list of DCT sizesshould be scanned in ascending order until a difference of greater thanor equal to 32 is found. The DCT number should then be renormalized toremove the wrap-around.

The differential delay between two channels A and B where DCT_(A)>DCT_(B) is:

    (DCT.sub.A -DCT.sub.B)×3ON+OFFSET.sub.A -OFFSET.sub.B +CID.sub.A -CID.sub.B

This expression provides the amount that B must be delayed to equalizeit with A. Conceptually the difference in DCT gives us the number ofsuper-frames apart, the offset gives us number of frames apart and thenwe need to correct for the difference in channel numbers (since theoverhead on channel 0 appears a frame before that on channel 1 etc.).

In equalizing delays it is first necessary to find the slowest channel(the one with the largest value of DCB+OFFSET-CID). This channel's delaybuffer is then set to zero and the delay of all other channelsdetermined relative to the slowest one.

In order to respond to frame slips it is desirable to set the slowestchannels delay buffer to some small non-zero value. This permits sliphandling of the channel. The choice of this value is implementationdependent, since it impact buffer size. In networks which sliprepeatedly this space will eventually be exhausted and then a completere-calibration will be required.

Circuit--Channel Number Mapping

In order to correct delay at the receiver the sequence in which therotation count is sent and the assignment of channel numbers to circuitnumbers must be known by the receiver.

The channel rotation count is sent first on the overhead channel andthen on the data channels in order of increasing channel number.

The channel numbers are assigned to the sender data circuits in thefollowing fashion:

in an ST-BUS the earlier timeslots get the lowest numbers

in MX streams simultaneous ST-BUS timeslots are ordered by PE slotnumber

in MX links the timeslots are ordered D1<CB1<D2<CB2

This protocol requires that data circuits from the data source tovirtual channel resource be mapped in the same fashion as the circuitsfrom the receiver VCR to data receiver. This ensures that data interfaceports are properly connected.

The circuits between VCRs need not be consistently mapped. The VirtualChannel Receiver will determine the channel number assignments and sendthe data out in increasing timeslot order on the data circuits to thedata receiver. This allows independence of the network.

The maximum delay resolution is a function of channel size and thenumber of bits used for the DCT. The worst case virtual channel is N=2.

The DCT bits occur with frequency 8N and it requires 6 frames to get acomplete DCT. Due to wrap-around of the counting we can only resolvedelays to one half of the maximum DCT value, or 32. The resultingresolution is:

    8×6N×32×0.125 μs

For N=2 this results in delay resolution of 384 ms and for N=7, 1344 ms.This precludes mixed satellite/terrestrial applications (it's unlikely aVCR would have enough buffer space for such applications).

In the described method data can be passed through a network in whichinput and output channels are randomly cross-connected with differentpath delays and re-ordered at the receiver.

A main advantage of the system described is that the method is adaptiveand will adjust for changes in delay during traffic transmission. Thesystem also allows a high bit rate synchronous channel to detect andrecover from changes in network (i.e. frame slips, protection switches)during the course of the connection and the signalling can be passed viaABCD bits in the overhead bytes.

We claim:
 1. A method of transmitting super-rate data signals having abit-rate higher than a predetermined bit-rate through a digital datatransmission system normally providing channels having saidpredetermined bit-rate, said channels being subject to differentpropagation delay characteristics through said system,comprising;allocating a group of k said channels (where k is an integer)in said data transmission system as a virtual channel for thetransmission of said super-rate signals; dividing said super-ratesignals into n sub-signals, where n is an integer and n≦k, having abit-rate equal to or less than said predetermined bit-rate; defining anoverhead channel in at least one of said channels; transmitting said nsub-signals over said channels; transmitting overhead signals atintervals in said channels in slots normally containing data signals,said overhead signals temporarily displacing the data signals normallyoccupying said slots; transmitting said displaced data signals over saidoverhead channel in slots normally occupied by the overhead signalsdisplacing them; and reassembling said n sub-signals to reconstitutesaid original super-rate data signals with the aid of said overheadsignals after transmission through said channels: wherein one of thechannels, which has a free bit position not required for transmittingdata, provides said overhead channel at a sub-rate of the predeterminedbit-rate, and the overhead signals are transmitted by insertingrespective individual bits thereof on a rotational basis into therespective data channels, each bit of the overhead signals beinginserted into the free bit position of said one channel or into data bitpositions of the other n-1 channels, the data bit that would normally besent in a bit position occupied by a data calibration bit being sent inthe free bit position of said one channel while its bit position isoccupied by an overhead bit.
 2. A method as claimed in claim 1, whereinsaid overhead signals are sent over an 8 kbs virtual overhead channelconsisting of said rotating bit positions distributed over said datachannels.
 3. A method as claimed in claim 1, wherein in a first framethe first overhead bit is sent in said free bit position of the firstchannel, in a second frame the second overhead bit is sent in apredetermined bit position of the second channel while the correspondingdata bit normally sent in that bit position is sent in said free bitposition of the first channel, in a third frame the third overhead bitis sent in a predetermined bit position of the third channel while thecorresponding data bit normally sent in that bit position is sent insaid free bit position of the first channel, and so on until an overheadbit has been substituted in all data channels, whereupon the cycle isrepeated.
 4. A method of transmitting super-rate data signals having abit-rate higher than a predetermined bit-rate through a digital datatransmission system normally providing channels having saidpredetermined bit-rate, said channels being subject to differentpropagation delay characteristics through said system,comprising:allocating a group of k said channels (where K is an integer)in said data transmission system as a virtual channel for thetransmission of said super-rate signals; dividing said super-ratesignals into n sub-signals, where n is an integer and n≦k, having abit-rate equal to or less than said predetermined bit-rate; generatingdelay calibration signals for transmission through said datatransmission system; defining an overhead channel in at least one ofsaid channels; transmitting said n sub-signals over said channels;transmitting said delay calibration signals in a rotational pattern oversaid channels in slots normally containing data signals, said delaycalibration signals temporarily displacing the data signals normallyoccupying said slots; transmitting said displaced data signals over saidoverhead channel in slots normally occupied by the delay calibrationsignals displacing them; and reassembling said n sub-signals toreconstitute said original super-rate data signals with the aid of saiddelay calibration signals after transmission through said channels.
 5. Amethod as claimed in claim 4, wherein the delay calibration signalsinclude rotation count bytes (LSBs).
 6. A method as claimed in claim 5,wherein said rotation count bytes (LSBs) are interspersed withmulti-purpose overhead bytes (OHB's) which carry overhead data for thevirtual channel.
 7. A method as claimed in claim 6, wherein apredetermined number of the most significant bits of the rotation countbytes (LSBs) are not used to carry rotation count information, andadditional bytes, referred to as (MSBs), which are interspersed with therotation count bytes (LSBs) and multi-purpose overhead bytes (OHBs), areused to carry the information missing from the unused bit positions ofthe LSB's.
 8. A method as claimed in claim 7, wherein the MSBs followeven-numbered LSBs and OHBs follow odd-numbered LSBs.
 9. A method asclaimed in claim 7, wherein said virtual channel is established betweena sender and a receiver and said multi-purpose overhead bytes comprisestatus words giving status information about the virtual channel to thesender, and channel id bytes indicating the channel numbers of thetransmitted virtual channel to the receiver.
 10. A method as claimed inclaim 4, wherein k=n+1 and one of said channels is reserved exclusivelyas an overhead channel dedicated to carrying, in a rotational pattern,said delay calibration signals and said data signals displaced by saiddelay calibration signals.
 11. A method as claimed in claim 4, whereink=n, and said overhead channel comprises a sub-rate channel formed byvacant bit positions in one of said channels having said predeterminedbit rate.
 12. In a digital data transmission system normally providingdiscrete channels each having a predetermined bit-rate, said channelsbeing subject to different propagation delay characteristics throughsaid system, an apparatus for transmitting super-rate data signalshaving a bit-rate higher than said predetermined bit-rate through saidchannels comprising:means for allocating a group of k said channels(where K is an integer) in said data transmission system as a virtualchannel for the transmission of said super-rate signals; means fordividing said super-rate signals into n subsignals, where n is aninteger and n≦k, having a bit-rate equal to or less than saidpredetermined bit-rate; means for generating delay calibration signalsfor transmission through said data transmission system; means fordefining an overhead channel in at least one of said channels; means fortransmitting said n sub-signals over said channels; means fortransmitting said delay calibration signals in a rotational pattern oversaid channels in slots normally containing data signals, said delaycalibration signals temporarily displacing the data signals normallyoccupying said slots; means for transmitting said displaced data signalsover said overhead channel in slots normally occupied by the delaycalibration signals displacing them; and means for reassembling said nsub-signals to reconstitute said original super-rate data signals withthe aid of said delay calibration signals after transmission throughsaid channels.
 13. An apparatus as claimed in claim 12, wherein sendingand receiving virtual channel resource units (VCRs) are providedrespectively at each end of said data transmission system, said sendingunit including means for inserting said delay calibration signals intosaid sub-signals in said rotational pattern and transferring thedisplaced data signals to the overhead channel, and said receiving VCRunit including a reframer to reassemble the received signals in the sameorder as said super-rate signal with the aid of said delay calibrationsignals.
 14. An apparatus as claimed in claim 13, wherein k=n+1 and oneof said channels is reserved exclusively as an overhead channeldedicated to carrying, in a rotational pattern, said delay calibrationsignals and said data signals displaced by said delay calibrationsignals.
 15. An apparatus as claimed in claim 13, wherein k=n, and saidoverhead channel comprises a sub-rate channel formed by vacant bitpositions in one of said channels having said predetermined bit rate.